Drag Sensitivity Feature:
Added a new dragSensitivity parameter to the RefreshIndicator, allowing developers to customize the amount of drag required to trigger a refresh. This feature provides greater control over the user experience, especially in applications where the default sensitivity may not be ideal.

Example Update:
Updated the refresh_indicator.0.dart example to demonstrate the new dragSensitivity parameter. The example now includes a sensitivity setting of 0.7, showcasing how this adjustment affects the refresh behavior.
List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.

Fixes #147363

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.