Allow customized coveragerc for Python coverage

### Description of the feature request:

For python coverage collection, `.coveragerc` is hardcoded in https://github.com/bazelbuild/bazel/blame/d435c6dd6e977a5c3ea1bc726557a9321948a317/tools/python/python_bootstrap_template.txt#L393-L397. To provide configurability, we'd like to introduce custom `.coveragerc` to set configurations including https://coverage.readthedocs.io/en/stable/excluding.html#advanced-exclusion.

### Which category does this issue belong to?

Configurability, Python Rules

### What underlying problem are you trying to solve with this feature?

Allow coverage configurations. In particular, we're interested in excluding import statements being collected as executable lines in coverage reports.

### Which operating system are you running Bazel on?

macOS, Linux

### What is the output of `bazel info release`?

release 6.3.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

首先阅读 tools/python/python_bootstrap_template.txt 第393-397行附近的内容，其中硬编码了 .coveragerc 路径，并跟踪 Python coverage 在 macOS 和 Linux 上的收集方式。完成的标准是，用户可以提供自定义的 .coveragerc 并配置 coverage 排除项（包括 import 语句），同时默认行为仍然可用。

由索引模型根据 Issue 内容生成。