Implement text_sensor based on ble_client

[ashald]

What does this implement/fix?

This PR implements text_sensor based on ble_client similarly to existing sensor component.

Currently, reading a string characteristic is possible via an obscure workaround:

ble_client:
  - mac_address: !secret ble_server_mac_address
    id: basement

text_sensor:
  - id: basement_last_activity
    platform: template

sensor:
  - name: "Ignored"
    platform: ble_client
    ble_client_id: basement
    service_uuid: !secret ble_service
    characteristic_uuid: !secret ble_characteristic_time
    lambda: |-
      std::string value(x.begin(), x.end());
      id(read_time).publish_state(value);
      return 0;

With this PR the above snippet can be simplified to what is show below.

Types of changes

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

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#1823

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

Example entry for config.yaml:

ble_client:
  - mac_address: !secret ble_server_mac_address
    id: basement

text_sensor:
  - id: basement_last_activity
    platform: ble_client
    ble_client_id: basement
    service_uuid: !secret ble_service
    characteristic_uuid: !secret ble_characteristic_time

Example log output

...
[01:14:52][C][ble_text_sensor:021]: BLE Text Sensor 'basement_last_activity'
[01:14:52][C][ble_text_sensor:022]:   MAC address        : ab:cd:de:00:11:22
[01:14:52][C][ble_text_sensor:023]:   Service UUID       : 9774321A-DEC1-461E-BD8D-D06902E9D950
[01:14:52][C][ble_text_sensor:024]:   Characteristic UUID: 64309E42-064B-4F39-80E4-A6DAD556CD5B
[01:14:52][C][ble_text_sensor:025]:   Descriptor UUID    : 00000000-0000-0000-0000-000000000000
[01:14:52][C][ble_text_sensor:026]:   Notifications      : NO
[01:14:52][C][ble_text_sensor:027]:   Update Interval: 60.0s
...
[01:15:23][D][text_sensor:067]: 'basement_last_activity': Sending state '2022-01-20T01:15:17'

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @buxtronix, mind taking a look at this pull request as it has been labeled with an integration (ble_client) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[buxtronix]

looks good.

[ashald]

@buxtronix thanks for the review!
What would be the next step to get it merged?
Thanks!

[ashald]

Is there anyone who can merge this PR?

[OttoWinter]

LGTM, thanks. Just one minor issue 👍