add ExpandedSignerList amendment support

[khancode]

High Level Overview of Change

Adds ExpandedSignerList amendment support.

Context of Change

• Expands the maximum signer list size to 32 entries.
• Adds an optional field wallet_locator to SignerEntry.
• rippled PR

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

Test Plan

Adds unit & integration tests.

Future Tasks

Merge this PR once the amendment goes live.

[JST5000]

LGTM after adding that test case :)

[intelliot]

• Expected error if someone tries to set too many signers (before amendment is enabled): temDISABLED
• Some test networks could activate the amendment before Mainnet
• Should be fine to merge and release at any time (so it's ready if/when the amendment activates)