Add support to custom extensions in streamable http client #2936

Job	Run time
checks / pre-commit	21s
checks / readme-snippets	10s
checks / test (3.10, lowest-direct, windows-latest)	1m 19s
checks / test (3.10, lowest-direct, ubuntu-latest)	34s
checks / test (3.10, highest, windows-latest)	1m 20s
checks / test (3.10, highest, ubuntu-latest)	34s
checks / test (3.11, lowest-direct, windows-latest)	1m 28s
checks / test (3.11, highest, windows-latest)	1m 26s
checks / test (3.11, lowest-direct, ubuntu-latest)	37s
checks / test (3.11, highest, ubuntu-latest)	34s
checks / test (3.12, lowest-direct, ubuntu-latest)	32s
checks / test (3.13, lowest-direct, windows-latest)	1m 38s
checks / test (3.12, lowest-direct, windows-latest)	2m 13s
checks / test (3.13, highest, windows-latest)	1m 41s
checks / test (3.12, highest, windows-latest)	1m 28s
checks / test (3.12, highest, ubuntu-latest)	36s
checks / test (3.13, lowest-direct, ubuntu-latest)	38s
checks / test (3.13, highest, ubuntu-latest)	35s
	17m 44s

Provide feedback