Adds support for prost-build's "extern_path" feature. This allows you to
reference another prost-generated protobuf, including any traits that
may have been defined for it, from another crate or location.
* chore: Add tags to .gitignore
When generating ctags/universal-ctags, a 'tags' file is generated. This
commit adds any generted 'tags' file to .gitignore.
* feat(build): Expose type_attribute and field_attribute
This commit exposes the `type_attribute` and `field_attribute`
configuration settings from Prost. These are useful to tweak/extend the
generated types.
For example:
```
tonic_build::configure()
.out_dir(tmp)
.format(false)
.type_attribute(".", "#[derive(Serialize, Deserialize)]")
.type_attribute(".", "#[serde(rename_all = \"camelCase\")]")
.field_attribute("in", "#[serde(rename = \"in\")]")
.compile(&["tests/protos/wellknown.proto"], &["tests/protos"])
.unwrap();
```
Would add the serde `Serialize` and `Deserialize` traits, while renaming
all the fields to camelCase, and having serde keep fields named `in`
named `in`, rather than Prost's `in_`, to every type generated by Prost.
