Optional-metadata-feild

[Jayesh45-master]

• The Jira issue number for this PR is: MDEV-______

MDEV-37674 Refactor Optional_metadata_fields storage

Replace std::vector-based Optional_metadata_fields members with a
more structured representation aligned with MariaDB internal types.

This refactoring simplifies memory allocation, reduces fragmentation,
and prepares the code for using Column_definition as the canonical
holder for column metadata.

No functional behavior is changed.

• The Jira issue number for this PR is: MDEV-37674

Description

Table_map_log_event::Optional_metadata_fields currently stores per-column
metadata using multiple std::vector members, one for each optional
attribute (types, flags, lengths, etc.).

This design leads to:

• Multiple independent allocations
• Complex indexing logic
• Redundant data structures
• Harder future maintenance

This PR refactors the internal representation to move away from
std::vector-heavy storage and align metadata handling with MariaDB
internal column structures. The change simplifies memory management,
makes the code easier to reason about, and prepares the ground for
using Column_definition as the single authoritative metadata holder.

This work was split out from MDEV-37674, which previously removed
std::string usage from the same area.

No functional behavior or binary log format is changed.

Release Notes

Internal refactoring of replication log event metadata handling.
No user-visible behavior changes.

How can this PR be tested?

This change is a structural refactoring with no functional behavior
changes. Existing replication and binlog test coverage exercises the
affected code paths.

Recommended validation:

• Run the existing replication test suite
• Verify binlog-based replication still functions correctly
• Confirm no changes in binlog parsing or event decoding behavior

No new tests were added since behavior remains unchanged.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[CLAassistant]

All committers have signed the CLA.