Encode Field and Schema metadata using the same type

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Right now, Schema metadata is encoded using HashMap and Field metadata is encoded using BTreeMap:

• https://docs.rs/arrow/19.0.0/arrow/datatypes/struct.Schema.html#method.metadata
• https://docs.rs/arrow/19.0.0/arrow/datatypes/struct.Field.html#method.metadata

This is confusing and makes for awkward code.

Describe the solution you'd like
A single unified metadata type for both Schema and Field

Describe alternatives you've considered

1. Standardize on either HashMap or BTreeMap
2. Standardize on either Option<HashMap> or Option<BTreeMap>
3. Make an arrow specific Metadata type that wraps HashMap / BTreeMap and implements whatever extra behavior might be needed

Additional context
There is quite a bit of discussion on #2261