Add BUILD_SHARED_LIBS option defaulting to ON

[wence-]

Description

This means that downstream libraries that get their fmt dependency from RMM will use -DFMT_SHARED in the compile command: this matches what the rapids combined devcontainers do, so we get sccache hits.

• Closes [FEA] Default to BUILD_SHARED_LIBS=ON in cmakelists #1701

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[wence-]

/merge