✨ Planned API endpoints

[JWCook]

This issue is for tracking overall progress on new API endpoints we currently plan to add. More can of course be added afterward, but these endpoints should cover the majority of iNaturalist features that most users will care about.

Once these issues and the issues listed #217 are complete, pyinaturalist will be ready for a 1.0 release.

• Add GET /messages endpoints from v1 API #79
• Add GET /users endpoints from v1 API #80
• Add GET /observations/observers endpoint from v1 API #88
• Add GET /observations/histogram endpoint from v1 API #90
• Add GET /observations{id}/taxon_summary endpoint from v1 API #243
• Add GET /observations/popular_field_values endpoint from v1 API #244
• Add create/update/delete observation endpoints from v1 API #103
• Add create/delete observation photo endpoints from v1 API #138
• Add create/delete observation sound endpoints from v1 API #229
• Add create/update/delete observation field value endpoints from v1 API #142
• Add GET /identifications endpoints from v1 API #104
• Add create/delete project observation endpoints from v1 API #139
• Add create/delete annotations endpoints from v2 API #140
• Add create/delete comments endpoints from v1 API #141
• Add sound file upload #153
• Add user life list endpoint #159
• Add taxon map layers endpoint #293
• Add undocumented PUT /projects/{id} endpoint from v1 API #343
• Add GET /users/me endpoint from v1 API  #471
• Support searching observations by observation fields #381