`MCO` configuration after `freeze`

[mayl]

As discussed on discourse, the STM32F1 family supports a feature named "MCO", which allows a clock to be routed to an external micro-controller pin. Which clock is turned on is controlled through MCO field of RCC_CFGR. Currently, stm32f1xx_hal does not expose a safe interface to this functionality because the RCC_CFGR is deliberately dropped on freeze. However, modifying this field of this register is safe and still upholds the static clock configuration contract of freeze.

Opening this issue here to track that a function to safely expose this functionality is desired, and solicit any additional feedback.