fix: introduce AbsolutePathBuf and resolve relative paths in config.toml (#7796)

This PR attempts to solve two problems by introducing a
`AbsolutePathBuf` type with a special deserializer:

- `AbsolutePathBuf` attempts to be a generally useful abstraction, as it
ensures, by constructing, that it represents a value that is an
absolute, normalized path, which is a stronger guarantee than an
arbitrary `PathBuf`.
- Values in `config.toml` that can be either an absolute or relative
path should be resolved against the folder containing the `config.toml`
in the relative path case. This PR makes this easy to support: the main
cost is ensuring `AbsolutePathBufGuard` is used inside
`deserialize_config_toml_with_base()`.

While `AbsolutePathBufGuard` may seem slightly distasteful because it
relies on thread-local storage, this seems much cleaner to me than using
than my various experiments with
https://docs.rs/serde/latest/serde/de/trait.DeserializeSeed.html.
Further, since the `deserialize()` method from the `Deserialize` trait
is not async, we do not really have to worry about the deserialization
work being spread across multiple threads in a way that would interfere
with `AbsolutePathBufGuard`.

To start, this PR introduces the use of `AbsolutePathBuf` in
`OtelTlsConfig`. Note how this simplifies `otel_provider.rs` because it
no longer requires `settings.codex_home` to be threaded through.
Furthermore, this sets us up better for a world where multiple
`config.toml` files from different folders could be loaded and then
merged together, as the absolutifying of the paths must be done against
the correct parent folder.

codex-rs/utils/absolute-path/Cargo.toml

@@ -0,0 +1,17 @@
+
+[package]
+name = "codex-utils-absolute-path"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+path-absolutize = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+
+[dev-dependencies]
+serde_json = { workspace = true }
+tempfile = { workspace = true }