YangTextSchemaSource is a CharSource

YangTextSchemaSource is a CharSource

YANG text is inherently a plain text, without the ability to detect the
actual encoding. This patch makes the baseline interface a CharSource,
pushing the control to users and defaulting to UTF_8.

This does not extend to YinTextSchemaSource, as XML provides for
explicit control over the character set that governs the document.

JIRA: YANGTOOLS-1508
Change-Id: I2875bb5850def25a0e9e4c9ce1921f5be3a5c77f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>