Add missing `MaxTerminationFeeExported` `InitialPledgeExported` and `GetBeneficiaryExported` miner methods

[akaladarshi]

There are some methods missing from the miner actor, but exist in the builtin actors:

• MaxTerminationFeeExported exist in the V16 bultin-actor, the parameter itself is exist, but the method is missing.
• InitialPledgeExported exist in the V16 builtin-actor
• GetBeneficiaryExported should exist from V10 to rest of the versions.

[sreenivasivbieb]

i will take over it

[rjan90]

Hey @sreenivasivbieb, are you planning to take on this issue ticket? If so, let me know, and I can assign it to you.

[sreenivasivbieb]

yeah pls
assign it to me @rjan90