Draft: Add support for receiving "Customized Monthly" schedule intervals

[bcantoni]

Add handling to recognize "Customized Monthly" as a possible interval type that can be returned for schedules.

Also add a test with a simulated xml response to confirm.

Fixes #1365

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tableauserverclient
__init__.py	7	0	100%
config.py	15	0	100%
datetime_helpers.py	25	1	1	96%
exponential_backoff.py	20	0	100%
filesys_helpers.py	31	0	100%
namespace.py	26	3	3	88%
tableauserverclient/bin
_version.py	358	212	212	41%
tableauserverclient/helpers
__init__.py	1	0	100%
headers.py	13	8	8	38%
logging.py	2	0	100%
strings.py	31	1	1	97%
tableauserverclient/models
__init__.py	45	0	100%
collection_item.py	42	7	7	83%
column_item.py	55	32	32	42%
connection_credentials.py	35	11	11	69%
connection_item.py	86	12	12	86%
custom_view_item.py	138	13	13	91%
data_acceleration_report_item.py	54	1	1	98%
data_alert_item.py	159	3	3	98%
data_freshness_policy_item.py	156	15	15	90%
database_item.py	207	36	36	83%
datasource_item.py	299	11	11	96%
dqw_item.py	104	5	5	95%
exceptions.py	4	0	100%
extract_item.py	45	4	4	91%
favorites_item.py	69	8	8	88%
fileupload_item.py	19	0	100%
flow_item.py	149	10	10	93%
flow_run_item.py	72	0	100%
group_item.py	84	2	2	98%
groupset_item.py	39	1	1	97%
interval_item.py	182	32	32	82%
job_item.py	188	10	10	95%
linked_tasks_item.py	80	1	1	99%
location_item.py	30	1	1	97%
metric_item.py	130	12	12	91%
oidc_item.py	64	2	2	97%
pagination_item.py	34	0	100%
permissions_item.py	111	12	12	89%
project_item.py	207	30	30	86%
property_decorators.py	100	18	18	82%
reference_item.py	23	1	1	96%
revision_item.py	60	0	100%
schedule_item.py	206	6	6	97%
server_info_item.py	37	4	4	89%
site_item.py	636	12	12	98%
subscription_item.py	101	2	2	98%
table_item.py	119	18	18	85%
tableau_auth.py	61	24	24	61%
tableau_types.py	27	1	1	96%
tag_item.py	15	0	100%
target.py	6	0	100%
task_item.py	57	2	2	96%
user_item.py	305	14	14	95%
view_item.py	220	15	15	93%
virtual_connection_item.py	64	8	8	88%
webhook_item.py	66	0	100%
workbook_item.py	362	16	16	96%
tableauserverclient/server
__init__.py	9	0	100%
exceptions.py	4	0	100%
filter.py	21	1	1	95%
pager.py	33	1	1	97%
query.py	143	18	18	87%
request_factory.py	1243	183	183	85%
request_options.py	373	5	5	99%
server.py	188	19	19	90%
sort.py	6	0	100%
tableauserverclient/server/endpoint
__init__.py	34	0	100%
auth_endpoint.py	75	10	10	87%
custom_views_endpoint.py	147	12	12	92%
data_acceleration_report_endpoint.py	21	0	100%
data_alert_endpoint.py	94	23	23	76%
databases_endpoint.py	111	30	30	73%
datasources_endpoint.py	330	34	34	90%
default_permissions_endpoint.py	44	3	3	93%
dqw_endpoint.py	45	16	16	64%
endpoint.py	187	17	17	91%
exceptions.py	70	6	6	91%
favorites_endpoint.py	95	22	22	77%
fileuploads_endpoint.py	51	0	100%
flow_runs_endpoint.py	62	9	9	85%
flow_task_endpoint.py	21	2	2	90%
flows_endpoint.py	211	54	54	74%
groups_endpoint.py	126	9	9	93%
groupsets_endpoint.py	72	7	7	90%
jobs_endpoint.py	68	9	9	87%
linked_tasks_endpoint.py	38	0	100%
metadata_endpoint.py	88	14	14	84%
metrics_endpoint.py	55	6	6	89%
oidc_endpoint.py	42	1	1	98%
permissions_endpoint.py	44	3	3	93%
projects_endpoint.py	150	24	24	84%
resource_tagger.py	127	36	36	72%
schedules_endpoint.py	108	11	11	90%
server_info_endpoint.py	36	10	10	72%
sites_endpoint.py	130	27	27	79%
subscriptions_endpoint.py	56	14	14	75%
tables_endpoint.py	110	36	36	67%
tasks_endpoint.py	63	6	6	90%
users_endpoint.py	134	16	16	88%
views_endpoint.py	140	9	9	94%
virtual_connections_endpoint.py	113	10	10	91%
webhooks_endpoint.py	54	9	9	83%
workbooks_endpoint.py	345	24	24	93%
TOTAL	11598	1383	88%