Support specifying compression level in wal_compression

I thought it makes sense to extend wal_compression to support compression
levels.

The patch replaces the simple enum-based setting with string-based
'method[:level]' syntax, similar to the --compress option in pg_dump.

What's inside:
- Unified GUC design: wal_compression = 'method[:level]'
- Supported formats: 'off', 'on', 'pglz', 'lz4[:level]', 'zstd[:level]'
- Algorithm-specific defaults: LZ4 defaults to level 1, ZSTD to level 3
when no level is specified.
- Parameter validation is performed at SET time.
- The string is parsed only once during GUC assignment.
- Backward compatibility for common settings: While the GUC type changes
from enum to string, common previous boolean-like string values (e.g.,
'off', 'true', '0') are handled, mapping to 'pglz' or 'off' correspondingly.
- Includes docs change proposal, as well extensive new regression and TAP
tests.

Additionally, it adds LZ4HC support -- for LZ4, if levels 10-12 are
specified, then, when available (checked at build time + runtime), it's
supposed to use LZ4HC for higher compression ratio. This part needs
additional testing.

Originally, I considered adding wal_compression_level but eventually
decided to exten wal_compression, because of two reasons:
1. with a separate param, a question of defaults needs to be solved, and I
didn't find an elegant solution;
2. "method:level" syntax is already used in another place – pg_dump

An early version of this patch was reviewed by Andrey Borodin off-list, and
it was extremely helpful.

looking forward to seeing feedback

Nik