Do not check schema for equality in concat_batches

[alamb]

Which issue does this PR close?

Closes #4800
Closes #4799

Rationale for this change

Per discussion on #4801, this PR stops validating that schemas of the concatenated batches are exactly equal and instead uses the provided schema

While this may hide some subtle bugs in downstream crates, it avoids arrow-rs enforcing an invariant (e.g. that schema equality also includes field metadata) that is not in other downstream crates

Note that mismatched types still generate a useful error message which should catch any egrious errors.

What changes are included in this PR?

1. Remove the check for schema equality
2. Update existing test
3. Add new test showing what happens with mismatched types

Are there any user-facing changes?

[alamb]

I plan to leave this open for another day or two in case anyone else has feedback on the approach