Fix input handling for encoding functions & various refactors (#18754)

## Which issue does this PR close?

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Part of #12725

## Rationale for this change

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Started refactoring encoding functions (`encode`, `decode`) to remove
user defined signature (per linked issue). However, discovered in the
process it was bugged in handling of certain inputs. For example on main
we get these errors:

```sql
DataFusion CLI v51.0.0
> select encode(arrow_cast(column1, 'LargeUtf8'), 'hex') from values ('a'), ('b');
Internal error: Function 'encode' returned value of type 'Utf8' while the following type was promised at planning time and expected: 'LargeUtf8'.
This issue was likely caused by a bug in DataFusion's code. Please help us to resolve this by filing a bug report in our issue tracker: https://github.com/apache/datafusion/issues
> select encode(arrow_cast(column1, 'LargeBinary'), 'hex') from values ('a'), ('b');
Internal error: Function 'encode' returned value of type 'Utf8' while the following type was promised at planning time and expected: 'LargeUtf8'.
This issue was likely caused by a bug in DataFusion's code. Please help us to resolve this by filing a bug report in our issue tracker: https://github.com/apache/datafusion/issues
> select encode(arrow_cast(column1, 'BinaryView'), 'hex') from values ('a'), ('b');
Error during planning: Execution error: Function 'encode' user-defined coercion failed with "Error during planning: 1st argument should be Utf8 or Binary or Null, got BinaryView" No function matches the given name and argument types 'encode(BinaryView, Utf8)'. You might need to add explicit type casts.
        Candidate functions:
        encode(UserDefined)
```

- LargeUtf8/LargeBinary array inputs are broken
- BinaryView input not supported (but Utf8View input is supported)

So went about fixing this input handling as well as doing various
refactors to try simplify the code.

(I also discovered #18746 in the process of this refactor).

## What changes are included in this PR?

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Refactor signatures away from user defined to the signature coercion
API; importantly, we now accept only binary inputs, letting string
inputs be coerced by type coercion. This simplifies the internal code of
encode/decode to only need to consider binary inputs, instead of
duplicating essentially exact code for string inputs (given for string
inputs we just grabbed the underlying bytes anyway)

Consolidating the inner functions used by encode/decode to try
simplify/inline where possible.

## Are these changes tested?

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Added new SLTs.

## Are there any user-facing changes?

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No.

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Author: Jefffrey