Refactor BLE connecting to allow scanning for vehicle presence

[Lenart12]

Description

This PR splits the ble.tryToConnect method in to two parts, by factoring out its first step in which we scan for a suitable tesla BLE advertisement (beacon). This is achieved by putting the code in a new ble.scanVehicleBeacon method along with other smaller refactorings. This allows us to expose the new method with ble.ScanVehicleBeacon.

The end user can then use a different (shorter) timeout to check for the presence of the vehicle before deciding if it wants to try to connect, since if a beacon got detected it would make sense to use a longer timeout to connect, or even do multiple retries, and if it wasn't detected allow for faster error handling.

Example

// Old way
conn, err := ble.NewConnection(ctx, vin)

// New way

// Either try to connect immediatly
conn, err := ble.NewConnection(ctx, vin)
// Or
scanCtx, scanCancel = context.withTimeout(ctx, 1*time.Second)
defer scanCancel()
beacon, err := ble.ScanVehicleBeacon(scanCtx, vin)
if err != nil {
  handleErr(err)
}
conn, err := ble.NewConnectionToBleTarget(ctx, vin, beacon)

Type of change

Please select all options that apply to this change:

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Checklist:

Confirm you have completed the following steps:

• My code follows the style of this project.
• I have performed a self-review of my code.
• I have made corresponding updates to the documentation.
• I have added/updated unit tests to cover my changes.