Support for adding hprof metadata into heap analysis

[pyricau]

At Square we upload the heap analysis results. Having extra general information about the state of the app can help (e.g. session token, navigation log, etc). All that information is available in the heap. Right now the only to get that is by parsing the hprof again post analysis. Instead, we should provide a hook in LeakCanary to add metadata, and retrieve it from the heap analysis itself.

We should probably stick to a Map<String, String> because HeapAnalysis needs to be serializable and printable as string.