Add API for adding Annotations on the timeline

[petethepig]

We should add ability for users to add annotations via an API.

For example, this is what those look like in grafana:

• [ ] We could start by having a simple API for creating these:

# this api is copied from graphite:
#  https://www.hostedgraphite.com/docs/api/annotations-and-events.html#creating-an-annotation-for-the-impatient

curl -X POST http://pyroscope:4040/api/v1/annotations -d "title=new deploy"

• [ ] then we could add something on the frontend to render these

[Rperry2174]

@petethepig should we merge this with #97 ?

[petethepig]

@Rperry2174 oops, my bad. we could have both I guess, yours could be the next step where a user can add their own annotations, and it can use the same api we implement in this issue.

[Rperry2174]

hey @petethepig until we get support for sql or something along those lines, should we just store this kind of information in badger?

[petethepig]

@Rperry2174 oh yeah, definitely

[Rperry2174]

Adding some similar apis for reference:

• https://docs.honeycomb.io/api/markers/
• https://github.com/honeycombio/honeymarker
• https://grafana.com/docs/grafana/v9.0/developers/http_api/annotations/