Add .tx_power property to _bleio.Connection

[tannewt]

It would allow changing the tx_power of an active connection.

[dhalbert]

https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s140.api.v6.1.1/group___b_l_e___g_a_p___f_u_n_c_t_i_o_n_s.html?cp=5_7_4_4_2_1_2_38#gad10e80ba20ddab42d1753fd1be2d8c26. The transmit power during a connection is set per connection.

[gbakerpennsy]

I would love to have this. Any type of support on this would be most helpful.