Read LSP message headers at once (#7449)

The current LSP headers reader implementation assumes a specific order
(i.e., `Content-Length` first, and then `Content-Type`). Unfortunately,
this assumption is not always valid, as no specification enforces the
rule. @caius and I encountered this issue while implementing the
Terraform LSP, where `Content-Type` comes first, breaking the
implementation in #6929.

This PR introduces a `read_headers` function, which asynchronously reads
the incoming pipe until the headers' delimiter (i.e., '\r\n\r\n'),
adding it to the message buffer, and returning an error when delimiter's
not found.

I added a few tests but only considered scenarios where headers are
delivered at once (which should be the case?). I'm unsure if this
suffices or if I should consider more scenarios; I would love to hear
others' opinions.


Release Notes:

- N/A

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Co-authored-by: Caius <caius@caius.name>