Introduce and use consistently APIOptions throughout (#323)

* new api options machinery ready in place

* WIP switching to new api options. client done

* WIP on the options, Database module done

* WIP on switching to new options, collections are missing

* wip in the Options switch: tests are missing

* options everywhere, tests working as well

* moving scalar defaults from api_options to defaults.py

* support for custom headers in all request-issuing objects

* export API options at higher level

* removed a spurious emb.h.prov. coercion test

Author: hemidactylus

CHANGES

@@ -1,9 +1,18 @@
 v 2.0.0
 =======
+Support for arbitrary 'database' and 'admin' headers throughout the object chain
+Reworked the options system:
+    - an APIOptions object inherited at each "spawn" operation, with overrides
+    - environment-dependent defaults if nothing supplied
+    - "spawn" operations expose top-level aliases for selected settings
+    - per-method overrides. For timeouts, `max_time_ms` becomes an alias for more specific timeout types
+    - collection and database's `info` method gets timeout parameters
+    - (not user-facing) classes in the hierarchy other than DataAPIClient have breaking changes in their constructor (now options-first and keyword-arg-only)
 Major restructuring of the codebase into directories
     - BaseCursor not exported by astrapy.cursors anymore
     - some imports (supposedly not used externally) changed slightly
     - adapted and limited scope of test_imports.py
+Removed AstraDBDatabaseAdmin's `from_api_endpoint` static method (reason: unused)
 Rename main branch from 'master' ==> `main`
 Cursors' state described by the `CursorState` enum
 Removal of several deprecated modules/features from previous versions:
@@ -15,6 +24,7 @@ Removal of several deprecated modules/features from previous versions:
     - 'caller_name' and 'caller_version' parameters where `callers` is now expected
     - 'id' and 'region' to DataAPIClient's 'get_database' (and async version). Use `api_endpoint` which is now the one positional parameter.
     - 'region' parameter of AstraDBDatabaseAdmin.get[_async]_database (was ignored already in the method)
+    - 'max_time_ms' parameter in AstraDBDatabaseAdmin's constructor and its `from_astra_db_admin` method (was already ignored in the body)
     - The 'max_time_ms' parameter in DataAPIClient's 'get_database' method.
     - Accordingly, the syntax `client[api_endpoint]` also does not accept a database ID anymore.
     - 'namespace' parameter of several methods of: DataAPIClient, admin objects, Database and Collection (use `keyspace`)

README.md

@@ -408,6 +408,18 @@ from astrapy.ids import (
 )
 ```
 
+API Options:
+
+```python
+from astrapy.api_options import (
+    APIOptions,
+    DataAPIURLOptions,
+    DevOpsAPIURLOptions,
+    PayloadTransformOptions,
+    TimeoutOptions,
+)
+```
+
 Result classes:
 
 ```python

astrapy/__init__.py

@@ -49,6 +49,7 @@ def get_version() -> str:
 __version__: str = get_version()
 
 
+from astrapy import api_options  # noqa: E402, F401
 from astrapy.admin import (  # noqa: E402
     AstraDBAdmin,
     AstraDBDatabaseAdmin,