Added support for brotli response body compression

[mmd-osm]

• Brotli enabled by default in CMakeLists (is this a good idea?)
• Adds a new package dependency: libbrotli-dev
• Compression level 5 seems to be a good trade-off between compression rate and processing time.
• Brotli is the preferred option for Accept-Encoding: gzip, compress, br http header, like on the Rails port/mod_brotli on Apache.
• Changeset upload decompression is not in scope, maybe we could add this later.

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Implementation details:

• cmake/FindBrotli.cmake was copied over from https://github.com/yhirose/cpp-httplib/blob/master/cmake/FindBrotli.cmake. It seemed like one of the more complete implementations that I could find out there.
• Brotli API calls in src/brotli.cpp were based on the implementation in https://github.com/yhirose/cpp-httplib/blob/master/httplib.h

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Closes #374

[mmd-osm]

@tomhughes : before merging this PR, it would be good to know, if libbrotli-dev is available on faffy. Although I don't see it listed in chef, the relevant files such as brotli/decode.h , libbrotlicommon.so.1 and libbrotlienc.so.1 might be pulled in by some other dependency already. TIA.

[tomhughes]

It was actually installed but I've added it to chef anyway.

[mmd-osm]

Thanks, I'll give it a try now. If there are some bugs during testing, we'll address them in another PR.

Edit: Looks good, CGImap 0.9.0.2403252213 replied with "content-encoding: br"