feat(client): escape_path_chars percent-encodes extra characters in path parameters

[tanmaykm]

Fixes #120.

Adds a client-level option that percent-encodes additional characters in path parameter values, on top of the standard RFC 3986 escaping:

client = Example.Client("https://api.example.com"; escape_path_chars = ".")
Example.getcustomer("acme.example.com-42"; client)
# GET /customers/acme%2Eexample%2Ecom-42

Some routers cannot match a literal . in a dynamic segment (Rails reads it as a format suffix and 404s), but do match %2E because they route on the raw path. RFC 3986 §2.3 makes both spellings the same identifier, so servers that decode before routing are unaffected.

Behaviour

• Default is empty, so existing clients send the same bytes as before.
• Applies to the values of every path parameter of every operation: scalars, array items, and object keys and values, for simple, label, and matrix styles, and for content-typed path parameters unless the encoder produced a pre-encoded value.
• Style delimiters (. for label, ;/= for matrix) and the parameter name from the path template are never touched.
• Composes with allowReserved: reserved characters still pass through, listed characters are encoded (a/b.c -> a/b%2Ec).
• % is rejected in escape_path_chars with an ArgumentError, since it would re-encode the percent-encoding itself.

Changes

• src/runtime.jl: Client.escape_path_chars field and constructor keyword; _percent_encode_chars; escape_chars threaded through _path_scalar, _path_array, _path_object, _path_parameter, and both path branches of _append_parameter!.
• test/runtime.jl: unit coverage for each style, delimiter preservation, allowReserved composition, and constructor validation.
• test/runtime_integration.jl: live HTTP check of the wire target with and without the option, and combined with allowReserved.
• docs/src/clients.md: new "Extra percent-encoding in path parameters" section.
• MIGRATION.md: maps the 0.2 pre_request_hook workaround to the new option.

Tests

• julia +1.12 --project=. -e 'using Pkg; Pkg.test()'
• julia +1.10.11 --project=. -e 'using Pkg; Pkg.test()'