Make TA time values configurable

[timbru]

Make the following configurable for users:

// Some timing constants used by the Trust Anchor code. We may need to support
// configuring these things instead..
pub const TA_CERTIFICATE_VALIDITY_YEARS: i32 = 100;
pub const TA_ISSUED_CERTIFICATE_VALIDITY_WEEKS: i64 = 52;
pub const TA_MFT_NEXT_UPDATE_WEEKS: i64 = 12;
pub const TA_SIGNED_MESSAGE_DAYS: i64 = 14;