Break down tasks in 3.1 Epic

[mpg]

The 3.1 Epic currently has one "size:L" tasks that needs to be studied and broken down in smaller tasks in order to make them more actionable and easier to estimate in the meantime.

size:M tasks should also be reviewed for any opportunity of breaking them down in smaller tasks.

[mpg]

Note: the main purpose of this task is to study #4683 and break it down in smaller, well-defined, actionable tasks. That means getting a high-level overview of uses of MBEDTLS_PRIVATE in the programs, and from there decide how it makes sense to split the task of remove (most of) them: could be by directory, by status (in some cases we may decide to make the field public again, in some others, to add accessors, in some others, to change what the program does, and in rare cases to keep using MBEDTLS_PRIVATE for test purposes), etc.