feat: add support for optional account in decoders

[atharmohammad]

1. Support for generating decoders with optional accounts, it considers 3 cases:

• Optional account in middle: These are used with PROGRAM_ID as placeholders to skip, can be omitted on user end.
• Optional accounts in the end: Accounts like Rent are omitted in some transactions and crashes indexer.
• Optional accounts marked with optional (In Legacy IDL) and IsOptional

2. Added next_account in account_utils for common utility functions.

3. Generated MPL Token Metadata decoder with latest IDL v1.14

4. Add tests in mpl-token-metadata-decoder for optional rent account in Instruction CreateMetadataV3.

fixes #334

[KellianDev]

Hey @atharmohammad, I'm reworking the decoder generation with a Codama renderer right now (see #386). I will use the next_account util you wrote and looking at the isOptional field of accounts on the Codama IDLs when generating the ArrangeAccounts implementation. So, ready to merge if the conflicts are fixed OR it will just be done once the CLI update is released 5 to 10 days out.

[KellianDev]

There's a small clippy issue left @atharmohammad, merging once CI goes through.

[KellianDev]

LGTM