Berry bytes add appendb64

[s-hadinger]

Description:

Add bytes().appendb64(to_be_encoded:bytes [, idx:int [, len:ind]]) -> bytes() to encode base64 and append to current bytes buffer. Optional index and len to encode only part of the buffer.

This is used to encode HTTP payloads in EventSource without allocating a new object at each message.

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.1.250203
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass