set size limit for binary cache

[Be-ing]

Is your feature request related to a problem? Please describe.
vcpkg adds to the binary cache continually without evicting old packages.

Proposed solution
Implement some way to specify the maximum cache size with an environment variable. I am not sure what the best way to do this would be. Setting a single size would not be sufficient because multiple cache sources can be specified in VCPKG_BINARY_SOURCES.

Describe alternatives you've considered
Periodically delete old packages server side, but there is no server for the default caching which just stores to the filesystem.

Additional context
Exceeding a size limit with a cache could require rebuilding all packages to get rid of old ones.

[autoantwort]

I thought of placing a file in the root of the filesystem binary cache. Do you simply want to specify a maximum size or maybe something like "up to 90% disk usage" or "until 5GB free space is left"

[autoantwort]

I have created microsoft/vcpkg-tool#945 for that