feat: request offer by e-mail

[coplat]

Description of change

Adds the option to deliver credential offers by e-mail, as opposed to only B2B to a target URL. This introduces a new DeliveryMethod enum to the SendOfferEndpointRequest struct and SendCredentialOffer commands. When a recipient_email is provided in the delivery method, the event CredentialOfferEmailSent is published to NATS which handles the email delivery.

Links to any relevant issues

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How the change has been tested

Describe the tests that you ran to verify your changes.
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Definition of Done checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have successfully tested this change in a docker environment

[codecov-commenter]

Codecov Report

❌ Patch coverage is 26.66667% with 44 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
agent_issuance/src/offer/aggregate.rs	11.42%	31 Missing ⚠️
agent_issuance/src/offer/views/mod.rs	0.00%	8 Missing ⚠️
agent_api_rest/src/issuance/offers/send.rs	0.00%	5 Missing ⚠️

Files with missing lines	Coverage Δ
agent_api_rest/src/issuance/credentials.rs	88.23% <100.00%> (+0.15%)	⬆️
agent_holder/src/offer/aggregate.rs	88.46% <100.00%> (+0.30%)	⬆️
agent_issuance/src/offer/event.rs	50.00% <ø> (ø)
agent_shared/src/config/mod.rs	92.37% <ø> (ø)
agent_api_rest/src/issuance/offers/send.rs	0.00% <0.00%> (ø)
agent_issuance/src/offer/views/mod.rs	71.60% <0.00%> (-7.85%)	⬇️
agent_issuance/src/offer/aggregate.rs	75.74% <11.42%> (-4.34%)	⬇️

... and 2 files with indirect coverage changes

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