[Feature]: Convert metadata homepage from a single URL into a dict

[hofbi]

Right now the metadata.json has a single key to specify the homepage. While most people use this to specify the github repo URL, others such as the bazel-lib link to the docs or boost linking to their websites.

Looking at other package sources such as pypi, there is the option to specify multiple project links such as the repositry, the docs, or the release notes. To me, this seems to be a nice feature for BCR as well.

This issue is related to #131.

[hofbi]

@Wyverald FYI we discussed about this on the last Build Meetup.