WebServer: improve speed of serving static files

[AndreyDodonov-EH]

Summary

In a webserver, in http_server_http1.c, in handle_http1_static_fs_resource:

STATIC_FS_RESPONSE_SIZE is calculated as a minimum amount of bytes needed for the header.
So buffer of the same size is then used to send file itself.
Currently it is 198 bytes, which results in a pretty slow file serving.

Describe the solution you'd like

Create a KConfig entry for that, e.g. CONFIG_HTTP_SERVER_STATIC_FS_RESPONSE_SIZE,
and, if its not equal to 0 (alternatively also if it is bigger than this calculated value), use it instead of STATIC_FS_RESPONSE_SIZE
Suggested fix:
#92971

Or we could rename STATIC_FS_RESPONSE_SIZE to STATIC_FS_RESPONSE_HEADER_SIZE, because that's what its, let it be calculated as it is now, and then use a separatre buffer of size CONFIG_HTTP_SERVER_STATIC_FS_RESPONSE_SIZE

[rlubos]

I'm not sure if we need two separate buffers, we'd need to allocate the file buffer anyway so why not use it for headers as well. Perhaps a static assert could validate that the buffer is large enough to accommodate headers.

I think we could simply have the buffer size configurable with Kconfig as you suggest, although since the buffer is currently allocated on stack, I think it should be mentioned in the help string that setting large buffer size will require higher stack as well.

Are you planning to work on this?

[AndreyDodonov-EH]

@rlubos
sure, here is the PR #92971