Added RSSI sensor

[muxa]

This adds ability to add a signal strength sensor for the BLE connection (for feature request https://github.com/dmamontov/esphome-blekeyboard/issues/25). The sensor is not exposed by default and can be added via rssi: configuration, e.g:

ble_keyboard:
  ...
  rssi:
    name: "RSSI"

[sourcery-ai[bot]]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.40%.

Quality metrics	Before	After	Change
Complexity	1.38 ⭐	1.35 ⭐	-0.03 👍
Method Length	177.62 😞	172.29 😞	-5.33 👍
Working memory	12.84 😞	14.72 😞	1.88 👎
Quality	63.71% 🙂	63.31% 🙂	-0.40% 👎

Other metrics	Before	After	Change
Lines	1044	1078	34

Changed files	Quality Before	Quality After	Quality Change
components/ble_keyboard/__init__.py	74.47% 🙂	74.93% 🙂	0.46% 👍
components/ble_keyboard/const.py	35.25% 😞	34.08% 😞	-1.17% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
components/ble_keyboard/__init__.py	ble_keyboard_press_to_code	2 ⭐	125 😞	8 🙂	68.04% 🙂	Try splitting into smaller methods
components/ble_keyboard/__init__.py	ble_keyboard_combination_to_code	0 ⭐	132 😞	8 🙂	69.20% 🙂	Try splitting into smaller methods

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