Add backends and simulated TPM device

Signed-off-by: Ettore Di Giacinto <edigiacinto@suse.com>

config.go

@@ -0,0 +1,40 @@
+package tpm
+
+import "github.com/google/go-attestation/attest"
+
+type Config struct {
+	emulated       bool
+	commandChannel attest.CommandChannelTPM20
+	seed           int64
+}
+
+type Option func(c *Config) error
+
+var Emulated Option = func(c *Config) error {
+	c.emulated = true
+	return nil
+}
+
+func WithSeed(s int64) Option {
+	return func(c *Config) error {
+		c.seed = s
+		return nil
+	}
+}
+
+func WithCommandChannel(cc attest.CommandChannelTPM20) Option {
+	return func(c *Config) error {
+		c.commandChannel = cc
+		return nil
+	}
+}
+
+func (c *Config) Apply(opts ...Option) error {
+	for _, o := range opts {
+		if err := o(c); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}